1. Field of the Invention
The invention relates generally to spread spectrum conferencing communication systems for providing an anti-jam voice communication system with a conferencing capability, and more particularly to a multiple channel system for receiving and demodulating two or more spread spectrum messages simultaneously. The invention also relates to a PSK communication system wherein a PN sequence is utilized as the spreading sequence and wherein the presence of each incoming message is detected by means of a digital transversal correlator.
2. Description of the Prior Art
Spread spectrum communication techniques are assuming greater significance as a means for providing secure communications. While a number of systems of the prior art relate to spread spectrum communications, such prior art systems do not provide a conferencing capability, i.e., the simultaneous decoding of two or more messages in a spread spectrum environment. The combination of an anti-jam (AJ) transmission and conferencing is of particular significance to a typical aircraft squadron in a tactical environment wherein two to six voice conversations may be required at any given time. This scenario provides a large number of potential users and an inherent randomness of calls, which would serve to eliminate an addressable system approach. The present invention utilizes a single PN sequence (either linear or nonlinear) which is generated by each system user modulator. Each participating user demodulator locks to a given message and provides a voice output. Incoming messages are identified, assigned a particular time slot, and assigned to a particular channel only. The channels are then combined at the audio level to provide a conferenced message. The invention is particularly advantageous in connection with a digital PSK (phase shift keyed) receiver. PSK transmission per se is well known and consists of the biphase modulation of a subcarrier frequency with digital data (zeros and ones) and transmission of a carrier phase modulated by such signal to a receiver where either coherent or non-coherent detection is employed to retrieve the subcarrier. Synchronization is achieved for proper demodulation in coherent PSK systems by use of a unique code having correlation properties suitable for providing this demodulation reference. One such well known code is the pseudonoise (PN code), a pseudorandom binary sequence. A number of references to this technique are provided by U.S. Pat. Nos. 3,402,265 and 3,305,636. U.S. Pat. No. 3,351,859 discloses a communications system wherein quasi-random noise modulation is used to distinguish identical signals successively received, i.e., identical PN sequences are received at different times or phases. U.S. Pat No. 3,432,619 discloses the assignment of a special PN sequence or part thereof to each system subscriber, but does not provide for conferencing the subscribers. U.S. Pat. No. 3,475,558 discloses a time gated PN multiplexing system for selection of one subscriber over another. Spread spectrum systems employ a bandwidth much greater than the information or data bandwidth, using a PN code independent of the data, but uniquely representative of the transmitter. At the receiver, all signals having a modulation format different from the transmitted format are discriminated against, since the correlation at the receiver provides a processing gain, i.e., an improved signal to noise ratio proportional to the ratio of signal bandwidth to data bandwidth.